Modern electronic prepress operations utilize laser scanning systems to write or record images for subsequent reproduction or to scan a prerecorded image at a predefined resolution rate. Such scanning systems may write or record images or scan prerecorded images on various prepress media including, photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials mounted onto an image recording surface or photo or thermal sensitive paper, polymer film or aluminum base printing plate materials, all used in electronic image reproduction. Such media are mounted onto a recording surface which may be planar but which is more typically curved and scanned with a recording or scanning beam. The primary components of such a system include a recording surface, usually a drum cylinder and a scan mechanism disposed and movable within the drum cylinder. The system also includes a processor, with an associated storage device, for controlling the scanning mechanism and for scanning a prerecorded image, a photodetector and detector processor. The processor and associated storage device may be housed within the system itself or separate from the system with appropriate interconnection to the system.
The processor, in accordance with stored programming instructions, controls the scanning mechanism to write or read images on the plate or other medium mounted to the inner drum cylinder wall by scanning one or more optical beams over the inside circumference of the drum cylinder while the drum cylinder itself remains fixed.
The scanning and hence the recording are performed over only a portion of the cylinder inner circumference, typically between 120.degree. and 320.degree. of the circumference of the drum cylinder. The optical beam(s) are typically emitted so as to be parallel with a central axis of the cylinder and are deflected, by for example, a spinning mirror, Hologon or Penta-prism deflector so as to form a single scan line or multiple scan lines which simultaneously impinge upon the recording surface. The deflector is spun or rotated by a motor about an axis of rotation substantially coincident with the central axis of the drum cylinder. To increase the recording speed, the speed of rotation of the beam deflecting device can be increased. To even further increase the recording speed, multiple beam scanning has been previously proposed.
One such proposed multiple beam scanner has utilized a spinning dove prism with a single light source, as discussed, for example, in U.S. Pat. No. 5,214,528. Using a dove prism beneficially allows the use of a multiple beam source, e.g. a laser diode array, while eliminating the need for multiple beam correction elements and associated hardware. Additionally, for reasons which need not be discussed here, the scan speed of multiple beam systems using a dove prism can exceed that of other types of proposed multi-beam systems.
As shown in FIG. 10A, a dove prism 116 is disposed in the optical path between a laser source array 110 and the spin mirror 130. The prism 116 rotates about an axis coincident with the longitudinal axis of the drum cylinder 140 (or an optical axis which becomes coincident) at half the speed of the spin mirror 130 to scan the image surface 135 of the drum 140. Since the spinning of dove prism 116 produces a 2.times.axial rotation of all light beams passing through the prism, the multiple beams leaving the prism will rotate in lockstep with the rotation of the spin mirror 130, as shown by the rays marked with filled circles in FIG. 10B. Accordingly, by passing the multiple light beams through a spinning dove prism, crossing of the multiple scan lines formed by the spin mirror is avoided.
Notwithstanding the type of scanning element being utilized, it is of primary importance that the light beam(s) contact the deflector as close as possible to a desired location to ensure that the appropriate scan line(s) are formed on the recording surface and hence the desired image is properly recorded. In the case of a multiple beam system, this includes maintaining the desired spacing or overlapping relationship of the simultaneously scanned beams with respect to each other and the reduction or elimination of any differential scan line bow between successive scan lines.
Further, if a light beam is rotated by a rotating dove prism, the beam's rotation and hence the rotation of the prism must be synchronized with the angular position of the rotating deflector to obtain a proper scan of the recording surface and thereby properly record the desired image. Small changes in the phase locking of the two motors, i.e. the prism and deflector motors, can create banding groups, particularly for commonly used laser array geometries.
A wobble in the spinning dove prism motor or other types of anomalies will cause a misalignment of the dove prism and can create significant banding artifacts which repeat every two scan passes of the deflector. If such a misalignment exists in multiple beam systems of the dove prism type as have been proposed, the system is restricted to recording during only every other rotation of the spin mirror to obtain high quality results. A four beam system is accordingly only two times faster than single beam system, an eight beam system only four times faster, and so on.
The effect of dove prism wobble on system banding can be reduced by a ratio of the beam diameter at the prism to the beam diameter at the spin mirror, e.g. at least a 20.times.-30.times. reduction is obtainable for prism beam diameters of .about.1 mm or less. Hence, by increasing the ratio, the contribution of prism wobble to system banding can be reduced significantly. Angular alignment sensitivities for scan line bow are improved by the same ratios. A small beam diameter in the prism also reduces the size of the required prism and prism motor.
Hence, banding and twinning caused by misalignment of the dove prism can significantly reduce the advantages which are otherwise obtainable from multi-beam, dove prism type scanning systems.
U.S. Pat. No. 5,097,351 proposes a multi-beam system which utilizes a controlled moveable reflector in lieu of a dove prism and requires each of two laser beams to follow a separate optical path, each having separate focusing and collimating lenses and an acousto-optical modulator (AOM). The controlled reflector is disposed in only one of the optical paths and is driven to rotate one beam in sync with the rotation of the spin deflector. Errors are detected, and corrected by driving the deflector to adjust angular alignment during recording operations. However, the complexity of the system makes implementation difficult, if not impractical.
The above referenced related application describes a multi-beam system which utilizes a spinning dove prism through which all of the writing beams are passed. As described in the application misalignments, caused for example by wobble about the rotation axis of the dove prism, are corrected utilizing a correction lens assembly. The correction lens assembly is controlled to move, e.g. in a cyclic fashion, during recording operations to correct detected misalignment errors.
In order to correct for such misalignments the correction lens assembly must be capable of moving in the manner described in detail in the above referenced related application, which is incorporated herein by reference. Because the lens may need to move dynamically, i.e. continuously, during writing operations, vibrations may be transferred to other system components or structures. Additionally, it may be beneficial to encode the actual movement of the correction lens to correct beam misalignment and/or to monitor movement of the lens during writing operations to ensure correction of the misalignment.